Many aircraft wings are designed using conventional airfoils. With a conventional airfoil, the upper and lower surfaces come together at a blunt or rounded leading edge (LE) and at a sharp trailing edge (TE).
Conventional airfoils are also used for transonic wings (i.e., wings designed for transonic flight). Transonic flight occurs when the airflow velocity over an aircraft is a mixture of subsonic flow (i.e., flow velocity less than the speed of sound) and supersonic flow (i.e. flow velocity greater than the speed of sound). Air flowing over the upper surface of a wing is accelerated by the upper surface curvature used to produce lift. As a result, the speed of the aircraft at which a portion of the airflow over the aircraft reaches the speed of sound (i.e., becomes sonic) may be considerably less than Mach one.
Briefly, the Mach number is the ratio of the aircraft's airspeed to the speed of sound at the aircraft's current altitude. Mach 1 occurs when the aircraft is flying at the speed of sound. The critical Mach number (Mcrit) is the Mach number of the aircraft's airspeed at which the airflow at some place along the aircraft reaches the speed of sound.
When the airflow over any portion of the aircraft does reach the speed of sound, a shock wave may be generated at that point. If the aircraft's Mach number increases above the critical Mach number, supersonic flow may be created over both the upper and lower surfaces of the airfoil resulting in the generation of shock waves at each of the airfoil locations. At transonic speeds, there are often several localized areas of supersonic flow delimited by shock waves.
Across a shock, the pressure and density of air increases significantly resulting in non-isentropic or irrecoverable losses that are classified as wave drag. As the Mach number of the aircraft is increased, a dramatic and abrupt increase in drag occurs which is referred to as the transonic drag rise. A shock wave slows the airflow and thus increases pressure leading to an adverse pressure gradient across the shock wave. Depending on the strength of the shock wave, the adverse pressure gradient may cause a localized separation of the airflow from the surface of the airfoil at the base of the shock wave. During transonic flight, shock waves and shock-induced boundary layer separation are consistent and significant sources of an aircraft's total drag.
The Mach number at which the transonic drag begins to substantially increase is known as the “drag-divergence Mach number” (Mdd). Because slight increases in the aircraft's Mach number beyond the drag-divergence Mach number can lead to significant increases in the drag on the aircraft, operating at such conditions is not usually economically practical.
To push the transonic drag rise toward higher Mach numbers and thus reduce wave drag at a given transonic speed, several methods have been employed. Some of the more common methods include using highly swept wings which can be relatively costly to manufacture, thin airfoils, and aft-camber airfoils. Supercritical airfoils have been created with higher critical Mach numbers. Supercritical airfoils typically have flattened upper surfaces to reduce flow acceleration and a highly cambered aft section to generate a significant portion of the lift. The aft-loaded wings shift the center of lift back resulting in larger nose-down pitching moments. Ultimately, an increase in nose-down pitching moments requires that both the wing and the horizontal tail work harder to trim the aircraft in flight. The drag associated with trimming the vehicle is referred to as trim drag. A larger nose-down pitching moment typically increases trim drag.
There is a limit to how thin a practical airfoil can be due to considerations other than aerodynamics. For example, thinner wings provide less fuel capacity. Moreover, the use of thinner airfoils usually increases the overall weight of the wing because thinner wings have shallower structural boxes.
Larger wings can also be used to increase the drag-divergence Mach number and thus reduce wave drag for a given transonic airspeed. With a larger wing area, airfoils having lower lift coefficients may be used, which in turn leads to less wave drag. However, the increased wetted area of a larger wing usually increases the wing's skin friction drag to such an extent that the additional skin friction drag offsets or outweighs any wave drag reductions.
U.S. Pat. No. 6,293,497 entitled “Airplane with Unswept Slotted Cruise Wing Airfoil” discloses an unswept, or substantially unswept, wing that employs slotted cruise airfoil technology to achieve higher cruise speeds comparable with that of swept un-slotted aircraft wings and to achieve higher lift at lower speeds. The contents of U.S. Pat. No. 6,293,497 are incorporated herein by reference in their entirety as if fully set forth herein.